Piping and Storage Tanks
The simplest way to channel water from the downspouts to a storage tank is with a dry-pipe system, in which the collected water is piped into an opening in the top of a storage tank. With this method, the pipes contain no water unless it’s actually raining. Dry-pipe systems are simple to set up but have the disadvantage of requiring a separate tank for each downspout.
Wet-pipe systems, on the other hand, make it possible to use a single large remotely located tank. A series of individual PVC downspout risers lead to a common underground fill pipe, which emerges next to the tank and enters it near the top. The underground fill pipe works something like a P-trap, filling with collected rainwater until it overflows into the tank.
For a wet-pipe system to work properly, the tank must be sited so that its inlet lies below the top of the lowest riser. To prevent the water trapped in the buried fill pipe from becoming stagnant, we install a slow-drip flow-control valve in the low point of the pipe to the tank, which allows it to drain between storms. A cleanout fitting next to the dripper valve provides access to the pipe should it be needed.
Tank options. Storage tanks are made from a variety of materials — including concrete, wood, and even a waterproof fabric like that used for waterbeds — but the three most common are food-grade polyethylene, fiberglass, and corrugated steel lined with plastic.
Polyethylene tanks are the least expensive option; they can hold up to 10,000 gallons. Fiberglass tanks cost more but have the advantage of being paintable and easily repaired if damaged. They are readily available in sizes up to 15,000 gallons. Many of our customers opt for costlier corrugated steel tanks, which look better than polyethylene or fiberglass and can be made to hold as much as 60,000 gallons.
In cases where aesthetics trump cost, we’ll have a local company custom-fabricate smooth steel tanks, coated with epoxy on the inside to prevent rust. Although freezing is not a problem in our area, we occasionally bury tanks to keep them out of sight.
An overflow pipe near the top of the tank prevents overfilling and provides a measure of air relief as the water level rises and falls. On fiberglass and polyethylene tanks larger than 5,000 gallons, we install a U-shaped relief vent to prevent the inrush of water from a large roof area from overwhelming the overflow, compressing the air in the tank, and blowing the top off. Air-relief vents are not necessary on corrugated tanks because their loosely fitted tops are not airtight.